A reduction in size and an increase in usable frequency range have been sought for mobile telecommunications devices such as cellular phones that have been dramatically widespread. Accordingly, a reduction in size and an increase in usable frequency range have been desired for electronic components used in the mobile telecommunications devices, too.
Some mobile telecommunications devices comprise a duplexer for switching between a transmission signal path and a receiver signal path, which allows a single antenna to be used for both transmission and reception. Such a duplexer comprises a transmission filter for allowing a transmission signal to pass therethrough and interrupting a reception signal and a reception filter for allowing a reception signal to pass therethrough and interrupting a transmission signal.
Surface acoustic wave filters have been recently used for the filters of some of the above-described duplexers. The surface acoustic wave filters have a feature that they are usable at frequencies up to 2 gigahertz and capable of being smaller in size compared to ceramic filters. However, if the future mobile telecommunications devices are designed to be used at frequencies of 2 gigaherz and higher, there are still many technical problems to be solved to make surface acoustic wave filters capable of being used at such high frequencies.
To solve these problems, attention has been given to devices called thin-film bulk acoustic resonators (that may be hereinafter referred to as FBAR) as disclosed in Published Japanese Patent Application 2000-278078. The FBAR is a thin-film piezoelectric resonator that utilizes the resonance of a piezoelectric thin-film in the direction of its thickness. The FBAR allows variations in resonance frequency by changing the thickness of the piezoelectric thin film. It is estimated that the FBAR is capable of being used at frequencies of several gigaherz.
One of the types of filters incorporating resonators is a ladder-type filter, for example. The ladder-type filter includes a series resonator and a parallel resonator as a basic configuration. The ladder-type filter is made up of a plurality of basic configurations that are cascade-connected as required.
In general, the duplexer is fabricated in such a manner that a chip including the transmission filter and a chip including the reception filter are made separately and these chips are mounted on a packaging board for the duplexer.
However, it is required to fabricate the two types of chips and to mount the two chips on the board when the above-mentioned method is employed. The problem is that the method requires a number of steps to manufacture the duplexer.